In recent years, the awareness of the earth's environmental problems has increased. In order to reduce the amount of carbon dioxide exhaust, interest has been growing in thermoelectric conversion devices that exploit the Seebeck effect for providing an electrical generating system that uses unutilized waste energy. The thermoelectric conversion devices are in commercial use as auxiliary batteries for interplanetary probes, and the effective use of this energy has an enormous potential because electrical power can be obtained, for example, from geothermal heat, the waste heat from factories, solar heat, the combustion heat of fossil fuels, and the like.
When applying a temperature gradient to both ends of semiconductor samples, a thermoelectromotive force that is proportional to the temperature gradient is generated. This is a phenomenon referred to as the Seebeck effect and the proportionality coefficient (the thermoelectromotive force per 1 K temperature gradient) is called the Seebeck coefficient. The performance of a thermoelectric conversion material is generally evaluated by using the dimensionless figure of merit ZT. When the Seebeck coefficient, which indicates the electromotive force of the thermoelectric conversion material at an absolute temperature T, is denoted by S, the electrical conductivity is denoted by σ, and the thermal conductivity is denoted by κ, the performance of the thermoelectric conversion material is represented by the dimensionless figure of merit ZT=T(S2σ/κ). The characteristics as a thermoelectric conversion material become increasingly superior with an increasing value for ZT.
In a thermoelectric conversion device, generally thermocouples are formed by p-type and n-type thermoelectric conversion materials that are bonded by a metal. These thermocouples are coupled and used in the form of modules in which thermocouples are connected serially in order to obtain the desired voltage. In consideration of the level of the conversion efficiency, the n-type and p-type thermoelectric conversion materials that are used in such thermoelectric conversion devices frequently use Bi2Te3 intermetallic compound single crystals or polycrystals. Although it is known that Bi2Te3 exhibits the highest thermoelectric conversion performance (ZT=1) in a temperature range near room temperature, since a large temperature gradient cannot be applied thereto, the conversion efficiency thereof as a power generating application is low, so that its application is nothing more than a cooling device for a portable refrigerator, for example, utilizing the Peltier effect.
Thanks to the fabrication of semiconductor quantum well in 1993 by a research group led by Dresselhaus et al. of the Massachusetts Institute of Technology in the US, a drastic improvement in the thermoelectric conversion performance was predicted theoretically (Non-patent Document 1), and partially demonstrated experimentally (Non-patent Document 2). According to the details of the theory, because the density of state is increased by confining the carriers in a quantum well (having a well width of around a few nanometers), the square of the Seebeck coefficient increases in inverse proportion to the well width.
Since Hicks et al. have proposed their theory, new ideas such as a multiple quantum well and a quantum dot superlattice have been proposed and several high performance thermoelectric conversion materials have been developed. For example, Venkatasubramanian et al. have succeeded in reducing largely the thermal conductivity almost without influencing on the electronic system by producing a Bi2Te3/Sb2The3 superlattice and have developed a thermoelectric conversion material having a ZT of up to 2.4 at room temperature (Non-patent Document 3). In addition, Harman et al. have achieved a ZT of up to 1.6 at room temperature by producing a PbSe0.98Te0.02/PbTe quantum dot superlattice (Non-patent Document 4). Further, Hsu et al. have found that in a bulk AgPbmSbTe2+m alloy, a quantum dot structure is formed and have achieved a ZT of up to 2.2 at 800 K (Non-patent Document 5).
However, since the above-described thermoelectric conversion materials contain rare heavy metal elements as main components, are easily decomposed at a high temperature of 200° C. or more and have high toxicity, they are apparently unsuitable for a power generating application at a high temperature around 1000 K at which a large converting efficiency can be expected.
Under such background, recently, high temperature thermoelectric conversion materials using metal oxides are actively developed. The metal oxide is the most stable form on the earth and many types of the metal oxide are thermally and chemically stable at a higher temperature around 1000 K. For example, SrTiO3 can be easily converted into an n-type semiconductor by substitutionally doping SrTiO3 with high-valence ions, such as Nb, and it is known that it has a ZT of up to 0.08 at room temperature and a ZT of up to 0.37 which is the highest with n-type metal oxides at 1000 K (Non-patent Document 6).
In addition, as a background art related to the above things, Patent Document 1 also should be referred to.
[Non-Patent Document 1]
L. D. Hicks and M. S. Dresselhaus, Phys. Rev. B47, 12727 (1993)
[Non-Patent Document 2]
M. S. Dresselhaus et al., Proceedings of the 16th International Conference on Thermoelectrics, 12 (1997)
[Non-Patent Document 3]
Venkatasubramanian, R., Siivola, E., Colpitts. T. & O'Quinn. B., Thin-film thermoelectric devices with high room-temperature figures of merit, Nature 413, 597-602 (2001)
[Non-Patent Document 4]
Harman, T. C., Taylor, P. J., Walsh, M. P. & LaForge, B. E., Quantum dot superlattice thermoelectric materials and devices, Science 297, 2229-2232 (2002)
[Non-Patent Document 5]
Hsu, K. F., Loo, S., Guo, F., Chen, W., Dyck, J. S., Uher, C., Hogan, T., Polychroniadis, E. K., & Kanatzidis, M. C., Cubic AgPbmSbTe2+m: Bulk thermoelectric materials with high figure of merit, Science 303, 818-821 (2004)
[Non-Patent Document 6]
Ohta, S., Nomura, T., Ohta, H., Hirano, M., Hosono, H. & Koumoto, K., Large thermoelectric performance of heavily Nb-doped SrTiO3 epitaxial film at high temperature, Appl. Phys. Lett. 87, 092108-092111 (2005)
[Non-Patent Document 7]
Keisuke Shibuya et al. Single crystal SrTiO3 field-effect transistor with an atomically flat amorphous CaHfO3 gate insulator, Applied Physics Letters Volume 85 Number 3 (2004)
[Patent Document 1]
Japanese Patent Application Publication No. JP-A-8-231223